1) Field of the Invention
This invention relates to an apparatus for measuring the unconfined yield strength of bulk granular materials and how it changes over time under different environmental conditions. The information produced by these measurements can determine how well a powder performs in various processes and equipment such as pharmaceutical tableting machines, pneumatics conveyors, container filling machines, drying systems, and catalyst towers.
2) Description of the Related Art
Many apparatus have been disclosed and produced that measure the unconfined yield strength of bulk granular materials. These systems range from simple cylindrical compaction cells to sophisticated shear testers and so called flow-no flow testers. The basic idea of the apparatus is to compress bulk granular material with a known pressure and then break the material to determine the strength it gained due to the compression. The known pressure is applied for a short time or over a long period to determine the time dependance of the strength measurement.
A typical apparatus for measuring unconfined yield strength is disclosed by Michael Rock and Jorg Schwedes in Powder Volume 157, June 2005. In the disclosed system, bulk material is compressed in a cylindrical mold that has been divided into two halves. During the compression, the two halves are held together by ring bands. After compression, the mold is removed and the freestanding material is broken by applying for to its top area. The main disadvantage of this approach is that sides of the mold must be manually removed to free the sample for the break test. In addition, the bottom of the mold must be blocked with a plug or flat plate to ensure sample does to flow from the bottom of the mold.
Another typical apparatus for measuring unconfined yield strength is disclosed by Trude Nysaeter and Gisle Enstad in Particle and Particle Systems Characterization Volume 24, June 2007. In the disclosed system, bulk material is compressed in a cylindrical mold. After compression, the mold is removed by holding the top of the sample in place and lifting the mold upward. The disadvantage of this approach is that the mold must be lifted up off of the sample slowly and carefully so the sample is not disturbed while force is maintained on the top of the sample to keep the sample in place.
Another apparatus for measuring unconfined yield strength is disclosed in U.S. Pat. No. 5,289,728. In the disclosed system, a sample of bulk material is compressed in a cylindrical mold with an opening at the bottom. During compression, the opening is blocked by a plug. After the sample has been compressed, the plug is removed and the sample is pushed through the bottom opening by pressure from above. The force required to push the sample from the mold is related to its unconfined yield strength. The disadvantage of this approach is that is expensive to automate and does not directly measure the unconfined yield strength of the sample.
Another typical apparatus for measuring unconfined yield strength is disclosed in U.S. Pat. No. 3,939,701. In the disclosed system, bulk granular material is loaded into cylindrical cavities and pressure is applied to the top of the cylinder to compress the powder. The cylinders are then rotated relative to each other and the resulting shear stress in the sample is measured. From this procedure the unconfined yield strength is calculated.
All of the disclosed apparatus for measuring unconfined yield strength suffer from one of the following problems, either they require a great deal of manual intervention or are complicated and expensive to automate and produce.